This invention relates to a weldable heat-curable sealant composition and its method of use to weld two pieces of metal together.
Certain adhesive and sealant applications such as those in the fabrication and assembly of automobiles require metal parts such as steel separated by a sealant to be welded together. The chemical compositions of the sealants are such that they harden or cure in place when the welded assembly is passed through the paint oven near the end of the manufacturing process. However, a useful thermosetting sealant must have enough cohesive strength in its uncured state to prevent it from sagging out of place or from being washed off by cleaning sprays.
Sealants which allow subsequent resistance welding are said to be weldable or to have weld-through capability. Weldable sealants are typically soft so that they can deform and allow the metal parts to touch during attempted welding. However, soft sealants have the disadvantage that they may fail other specifications such as strength, sag resistance, or resistance to cleaning sprays that are common to the industry. Therefore, it is desirable for weldable sealants to have as much viscosity, cohesive strength, tack and body as is required to meet such specifications. It is difficult to formulate a sealant which is both soft enough to be weldable and strong enough in its uncured state. In our experience, most sealants with viscosities below about 5 million centipoises at 25.degree. C. are weldable, but too weak, whereas most sealants with viscosities above about 5 million centipoises cannot be welded. Also, many sealants which are soft at the initial application temperature of 80.degree. C. to 100.degree. C. become so stiff after standing for a few hours at ambient temperature that they do not permit the two pieces of metal to come close enough together to form a good weld. A sealant may not be considered truly weldable unless it allows a good weld to be made after standing in place at ambient temperature for four days (96 hours).